This invention relates to constant bandwidth automatic gain control systems. More particularly, the invention relates to a simple, inexpensive constant bandwidth automatic gain control system for use in a servo system controlling the position of the read/write head of a rotatable magnetic disk storage system.
In a magnetic information storage system, magnetic media (on which information has been previously stored in the form of selectively magnetized particles) moves relative to a magnetic read head. Movement of the media past the read head may be realized by linearly moving a tape past a stationary head (a tape storage system), or by rotating a magnetic disk past a read head that is radially positioned with respect to the rotating disk so that selective tracks or bands on the disk can be accessed (a disk storage system). Each of the selectively magnetized particles on the media have a magnetic flux associated therewith. The movement of the magnetic flux past the read head induces a voltage therein representative of the magnetically stored information. This induced voltage is then amplified in order to generate a read signal of sufficient amplitude to be useful for correctly interpreting the stored information.
Unfortunately, the voltage induced in the read head may vary a great deal in amplitude. This amplitude variation may be caused by numerous factors, such as variations in the distance between the head and the media, the relative speed at which the media passes the head, impurities or other imperfections in the media or head, and the like. Such variations in amplitude, if not eliminated, can cause significant errors in the proper interpretation of the stored information. The problem is especially acute in a magnetic disk storage system, wherein servo bands are pre-recorded on a disk surface in order to position the read head at the correct radial position on the disk. In such a system, any variations in the amplitude of the induced servo positioning signal(s) may translate directly into positioning errors of the head. Therefore, some means must be used for maintaining the amplitude of the sensed signal at a constant level or amplitude even when the voltage induced in the read head may significantly vary in amplitude.
Automatic gain control (AGC) systems are well known in the art for maintaining an output signal at a constant amplitude regardless of variations in the input signal amplitude. See, e.g., Markus, Modern Electronic Circuits Reference Manual, Chapter 6 (Automatic Gain Control Circuits), McGraw-Hill Book Company (1980). Such systems, which are used for a wide variety of applications, typically include a variable gain element that controls the amplification of the amplitude-varying input signal so that the output signal maintains an amplitude that is more or less constant. These AGC systems are often realized by comparing the amplitude of the output signal to a reference signal, generating an error signal representing the difference between the output signal and the reference signal, and then using the error signal as a control signal to control the gain of the variable gain element. This gain is selected such that the input signal is amplified by a gain factor that yields an output signal of the desired amplitude.
While such AGC systems maintain a constant amplitude output signal, they unfortunately also exhibit a varying loop bandwidth. The loop bandwidth of an AGC system relates to how quickly or how slowly the system responds to a change in the input signal. If the bandwidth is too small, the system may respond much too slowly to a change in input signal level to satisfy the requirements of the desired application. If, on the other hand, the bandwidth is too large, the system response will not be properly optimized for disk defect immunity and stability requirements. For servo positioning applications, where the response time must be carefully selected, and where a stable system is required, the selecting and maintaining of a desired bandwidth is thus critically important.
Attempts have been made in the art, with various degrees of success, to provide a constant bandwidth AGC system. U.S. Pat. Nos. 3,684,974 and 4,326,226, are representative of such attempts. However, all such constant bandwidth AGC systems known to applicants are either ineffective for disk storage systems (that is, they don't achieve the needed gain at the needed bandwidth for a particular application), or their implementation requires rather sophisticated circuitry.